This invention relates to wireless communication systems, and more particularly to signal power control within wireless communication systems.
Cellular telephones may operate under a variety of standards including the code division multiple access (CDMA) cellular telephone communication system as described in TIA/EIA, IS-95, Mobile station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, published July 1993. CDMA is a technique for spread-spectrum multiple-access digital communications that creates channels through the use of unique code sequences. In CDMA systems, signals can be and are received in the presence of high levels of interference. The practical limit of signal reception depends on the channel conditions, but CDMA reception in the system described in the aforementioned IS-95 Standard can take place in the presence of interference that is 18 dB larger than the signal for a static channel. Typically, the system operates with a lower level of interference and dynamic channel conditions.
A mobile station using the CDMA standard constantly searches a Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than a threshold value. As the mobile station moves from the region covered by one base station to another, the mobile station promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the base station or base stations of the promotion from the Neighbor Set to the Candidate Set via a Pilot Strength Measurement Message. The base station determines an Active Set according to the Pilot Strength Measurement Message, and notifies the mobile station of the new Active Set via a Handoff Direction Message. When the mobile station commences communication with a new base station in the new Active Set before terminating communications with the old base station, a xe2x80x9csoft handoffxe2x80x9d has occurred.
One problem with a xe2x80x9csoft handoffxe2x80x9d is that RF resources are used inefficiently while mobile stations are in soft handoff. To solve this problem, some providers have proposed using fast cell selection mechanisms. However, such mechanisms have several problems, including an imbalance in the forward and reverse link, a selection feedback delay, and reduced reliability. What is desired is a system that includes the advantages of both the soft handoff and the fast cell selection mechanism while reducing the negative aspects of each system.
The present invention enhances the ability of a mobile station to complete handoff by maintaining channels on the secondary base stations at low levels. The mobile station searches all active base stations to determine if a lock on the dedicated channel can be maintained. If a lock can be maintained, the mobile station provides feedback to the base station to adjust the power of the secondary channels. The power is adjusted so that the secondary channels are maintained at a low power level, typically a barely receivable level.
One aspect of the invention is a method of controlling power in a wireless communication system comprising detecting a signal and instructing the transmitter to reduce the signal power if the signal power exceeds a predetermined threshold. The method also comprises instructing the transmitter to increase the signal power if the signal power does not exceed the predetermined threshold. The threshold may be, among other things, a receive signal strength indicator (RSSI), a bit error rate, or a finger correlation of a null or preamble signal.
Another aspect of the invention is a method of controlling the power level of a channel comprising selecting a power measurement metric, comparing the metric to a threshold value, and adjusting the power level of the channel based on the comparison. The channel may include usable data or may include null data.
Another aspect of the invention is a mobile station for use in a wireless communication system. The mobile station comprises a signal detector and a power adjustment circuit. The power adjustment circuit provides instructions on a power setting based on the detected signal. The power setting instructions are provided via a closed-loop feedback channel.